Log TLS handshake result

Makes it easier to debug TLS issues, and to see the effect of
altering the priority string.

common/rfb/CSecurityTLS.cxx

     throw AuthFailureException("TLS Handshake failed");
   }
 
+  vlog.debug("TLS handshake completed with %s",
+             gnutls_session_get_desc(session));
+
   checkSession();
 
   cc->setStreams(tlsis, tlsos);

common/rfb/SSecurityTLS.cxx

     throw AuthFailureException("TLS Handshake failed");
   }
 
-  vlog.debug("Handshake completed");
+  vlog.debug("TLS handshake completed with %s",
+             gnutls_session_get_desc(session));
 
   sc->setStreams(tlsis, tlsos);